What to Plant Along Retaining Walls in New Hampshire Hardscapes

Retaining walls are more than structural elements: they are vertical gardens waiting to be softened, animated, and integrated into the landscape. In New Hampshire, where winters are long, summers vary from cool to warm, and soils and exposures change from site to site, choosing the right plants requires attention to hardiness, root behavior, moisture, salt tolerance, and seasonality. This article provides practical, in-depth guidance to help you select, place, and care for plants that perform well on and around retaining walls in New Hampshire hardscapes.

Understanding New Hampshire Conditions

New Hampshire spans USDA hardiness zones roughly from 3b in the north and higher elevations to 6b in the southern lowlands. Local conditions on your site can vary substantially because of aspect, elevation, proximity to water, and urban microclimates.

• Cold tolerance: Select plants reliably hardy to your zone, ideally with a margin of one zone colder for safety.
• Exposure: South- and west-facing walls receive more sun and heat, which can cause drying and winter desiccation. North-facing walls are cooler and shadier, favoring shade-tolerant species.
• Soil and drainage: Retaining walls often sit where water collects or runs off. Drainage behind the wall is critical; plants should suit either moist, well-drained pockets in terraces or drier, fast-draining edges.
• Deer pressure and salt: Deer browse is common in much of New Hampshire. Roads and driveways may expose plants near the wall to deicing salts; choose salt-tolerant and deer-resistant selections where needed.

Design Principles for Retaining Wall Plantings

Successful plantings balance structure, maintenance, and seasonal interest. Consider the following principles before choosing species.

• Prioritize root behavior: Avoid trees with aggressive roots that could destabilize walls. Use shallow-rooted shrubs, perennials, and groundcovers that bind soil without forcing out masonry.
• Layer vertically: Use a combination of groundcovers, low shrubs, medium shrubs, and tall accents to create depth and hide hard edges.
• Match plant moisture needs to micro-sites: Dry-tolerant plants for exposed wall faces and crevices; moisture-loving plants for ledges near irrigation or water runoff.
• Consider scale and maintenance: Smaller, slower-growing species reduce the need for frequent pruning and minimize pressure on the wall.
• Use native or adapted species: They are more likely to thrive with lower inputs and support local ecology.

Plant Choices by Use and Position

Choosing the right plant depends on where it will go relative to the wall: top of wall, crevices and seams, face, toe (base), or terraces and steps. Below are recommended options tailored to New Hampshire conditions, divided by use.

Groundcovers and Creepers for Faces and Crevices

Groundcovers and small trailing plants are ideal for softening stones and preventing erosion from small pockets of soil.

• Sedum (Sedum spurium, Sedum album): Excellent for sunny, dry exposures and shallow pockets. Low maintenance, drought tolerant, and winter hardy.
• Creeping thyme (Thymus serpyllum): Fragrant, blooms attract pollinators, tolerates heat and shallow soils on sunlit walls.
• Moss and native rock cress (Arabis caucasica): For shaded north-facing walls, moss or low rock cresses establish in cool, damp crevices.
• Ajuga (Ajuga reptans): Useful for semi-shade; forms dense mats and tolerates compaction, but can spread and require containment.
• Saxifraga (Saxifraga spp.): Preferred in cooler, shaded crevices with consistent moisture; good for alpine-style walls.

Low to Medium Shrubs for Top and Terraces

Shrubs anchor the composition at the top of the wall, provide structure, and can screen views.

• Dwarf yews (Taxus x media ‘Densiformis’): Tolerant of pruning and winter exposure; deer may browse, so consider site-specific risk.
• Boxwood (Buxus microphylla or B. sempervirens cultivars hardy to zone 5/6): Use in sheltered locations; choose cold-hardy cultivars for New Hampshire.
• Dwarf hydrangeas (Hydrangea paniculata ‘Little Lime’, Hydrangea arborescens ‘Annabelle’): Provide summer blooms and do well in partial shade; plant in soil with good moisture retention.
• Rugosa rose (Rosa rugosa): Salt tolerant, deer less likely to browse heavily, provides season-long interest and hips for winter color.
• Inkberry holly (Ilex glabra): Evergreen, salt tolerant, native, and suitable for moist soils.

Ornamental Grasses and Perennials for Movement and Texture

Grasses add winter structure and tolerable root systems for wall-adjacent beds.

• Little bluestem (Schizachyrium scoparium): Native, clump-forming, drought tolerant, and provides fall color.
• Switchgrass (Panicum virgatum ‘Heavy Metal’ or ‘Shenandoah’): Taller accents for terraces; clumping habit reduces wall pressure.
• Hosta (Hosta spp.): For shady terraces and top-of-wall plantings; mulch well to prevent winter heaving.
• Heuchera (Heuchera spp.): Attractive foliage for edges and partial shade; many cultivars are cold-hardy.

Vines and Trailing Plants for Softening Tall Walls

Vines will dramatize a wall when chosen carefully. Opt for non-invasive species with manageable root systems.

• Climbing hydrangea (Hydrangea anomala petiolaris): Works on shaded walls where it can attach to rough masonry; slow to establish but long-lived.
• Vinca major and Vinca minor (periwinkle): Vinca minor for shade, forms evergreen mats; avoid Vinca major where it may be invasive.
• Clematis (Clematis integrifolia, Clematis viticella cultivars): Use on trellis or pergola-adjacent walls; deep-rooted but non-aggressive.

Small Trees and Tall Accents (Use with Caution)

Trees can be used sparingly, preferably above the wall with roots directed away from the structure, or in large terrace planters.

• Serviceberry (Amelanchier spp.): Small, multi-season interest and typically non-invasive roots.
• Dwarf Japanese maple (Acer palmatum ‘Beni Hime’ or ‘Kiyohime’): Use in sheltered, protected spots; shallow-rooted but generally nonthreatening to walls if placed appropriately.
• Crabapple (Malus spp. dwarf forms): For seasonal blooms and fruit; avoid large shade trees directly at the wall base.

Practical Planting and Maintenance Steps

A reliable installation process will protect the wall and set plants up for success.

1. Evaluate the micro-site: determine exposure, soil depth, drainage behind the wall, and deer or salt pressure.
2. Prepare pockets and planting medium: add well-draining, moderately fertile soil for pockets; blend native topsoil with compost and a small proportion of sand or grit for improved drainage.
3. Use geotextile or root barrier where necessary: prevent root intrusion into wall drainage systems and direct roots away from weep holes.
4. Place plants to allow mature spread: maintain appropriate distances so shrubs and groundcovers do not become root-stressed and force masonry.
5. Mulch conservatively: use a 1 to 2 inch layer of coarse mulch on top terraces to retain moisture and reduce freeze-thaw heaving. Keep mulch away from plant crowns and from direct contact with the wall in narrow crevices.
6. Water deeply and infrequently during establishment: supplemental irrigation for the first two growing seasons helps roots settle into shallow, rocky soils.
7. Prune and monitor: remove dead wood annually, shear only where appropriate, and monitor for wall movement or signs of root pressure.

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Seasonal Care and Winter Considerations

New Hampshire winters and freeze-thaw cycles can cause soil heaving, desiccation, and salt damage.

• Choose plants with winter desiccation tolerance for south- and west-facing walls: evergreens near exposed walls can lose needles or foliage if not selected carefully.
• Protect against salt: plant salt-tolerant species (Rosa rugosa, junipers, some ornamental grasses) near paths and driveways.
• Prevent winter heaving: plant slightly deeper than you would in a garden bed, and use insulating mulch through the first winter. Avoid excessive nitrogen late in the season which can promote tender growth.
• Cut back ornamental grasses in late winter to early spring before new growth begins, reducing wind damage and mass loss when snow slides.

Common Problems and Solutions

Recognizing common issues early prevents long-term damage to both plants and structures.

• Problem: Wall heaving or cracking after planting. Solution: Check root types; remove aggressive plants, improve drainage behind the wall, and consult a structural engineer for repair.
• Problem: Plants drying out on sun-exposed walls. Solution: Choose drought-tolerant species, add water-retentive soil pockets or micro-irrigation, and apply mulch to conserve moisture.
• Problem: Deer browsing new growth. Solution: Use deer-resistant plants, apply deterrents, or physically protect plants until established.
• Problem: Salt damage near roads. Solution: Select salt-tolerant plants and create a buffer (hardy shrubs and grasses) between the road and more sensitive plantings.

Design Ideas and Composition Tips

• Repetition and rhythm: Repeating a few species at regular intervals creates cohesion and helps hide the linear nature of a wall.
• Seasonal layering: Combine early spring bulbs planted in wall pockets with later-blooming perennials and grasses for continuous interest.
• Contrast textures: Pair fine-textured groundcovers and thyme between stones with bold-leaved hostas or heucheras on the terrace.
• Native pollinator patches: Integrate native perennials like Echinacea, Rudbeckia, and Monarda on terraces to support bees and butterflies.

Final Takeaways

• Match plant hardiness and moisture needs to micro-site conditions rather than guessing by region alone.
• Favor shallow-rooted, clumping, and non-invasive species in direct contact with walls; keep large trees well away from structural elements.
• Prepare planting pockets with good soil and drainage, use geotextiles where appropriate, and mulch to reduce heaving.
• Incorporate a mix of evergreens, shrubs, perennials, grasses, and groundcovers for year-round interest, erosion control, and reduced maintenance.
• Monitor plant growth and wall performance annually; intervene early on drainage or root issues to protect both masonry and plantings.

With careful species selection, correct placement, and modest maintenance, retaining walls in New Hampshire can become attractive, functional living features that enhance stability, biodiversity, and curb appeal for years to come.